1. Field of the Invention
The present invention relates to a DC motor, in particular to a motor used in an electrical power tool, etc.
2. Description of the Related Art
The motors used in electrical power tools, etc., are required to have various properties, such as high output, high torque, compactness, lightweight, and low power consumption, etc. In such a field for power electrical tools, a direct-current motor having a 4-pole field magnet has been devised (see, for example, International Patent Publication WO 08/126,834 and Japanese Patent Application Publication No. 2009-165213). Also, in the field for electrical power tools, there has been an increasing demand for a reduction in the total length of a motor in recent years.
When the total length of a motor is intended to be reduced while the performance of the motor is being maintained, the volume of a magnet is made to be reduced if the diameter of the motor is not changed. Accordingly, it is needed to adopt a magnet with high magnet force, such as a rare-earth magnet, in order to generate necessary magnetic field by a magnet having a smaller volume. In this case, a reduction in the cost of the motor cannot be achieved because the magnet with high magnetic force is expensive. Therefore, the present inventor is trying to adopt a low-cost magnet in a motor whose total length is reduced while whose performance necessary for electrical power tools is being maintained.
However, the life of such a motor sometimes becomes shorter than expected. For example, when a ferrite magnet is adopted in view of cost, the magnetic field generated by the magnet with a reduced volume becomes weak, and hence it is needed to enhance the magnetic field generated by an armature (rotor) when the performance of the motor is maintained. In this case, because the strength of the magnetic field generated by the magnet becomes relatively weaker than that of the magnetic field generated by the armature, an influence by a so-called armature reaction becomes large. It is believed that the life of the motor has become shorter than expected as a result of that.